The present invention relates to a temperature monitoring device and a thermocouple assembly therefor, particularly for use in a system for effecting annealing treatments, particularly for annealing embrittled reactor vessels.
During the normal operation of a nuclear reactor, the reactor vessel, which is normally made of steel and which houses a core containing nuclear fuel, is exposed to intense radiation. Experience has shown that this radiation causes changes in the fine grain structure of the steel walls of the vessel. These structural changes make the walls brittle, a problem commonly referred to as reactor vessel embrittlement. Embrittlement reduces the flexibility of the vessel wall and increases the susceptibility of the vessel wall to fracturing, particularly if subjected to sudden stresses, such as due to operating transient events and pressurized thermal shock events.
Because of this embrittlement phenomenon, the United States Nuclear Regulatory Commission requires that a reactor vessel be removed from service when embrittlement reaches a predetermined stage, thus ending the useful life of this portion of the nuclear power plant. Replacement of such a vessel is extremely expensive because the vessel is built into and is a part of the reactor containment building, thereby making replacement economically impractical.
In order to deal with this problem, it has been proposed to subject such a vessel to annealing in place in order to restore the ductility and toughness of the metal constituting the reactor vessel.
Such treatments can be carried out with the system disclosed in the above-cited copending application Ser. No. 07/368,456, using the heater unit assembly disclosed in the above-cited copending application Ser. No. 07/368,432.
Accurate control of the temperature in each region of the pressure vessel wall is essential to achievement of a proper annealing result. The accuracy of such temperature control is dependent on the accuracy with which the wall temperature can be monitored.
While thermocouples are preferred temperature monitoring devices, they inherently have a certain inaccuracy, which is accentuated in situations where a large temperature gradient exists between the heat sources and the surface being heated.
The temperature reading produced by a thermocouple is dependent on the actual temperature of the thermocouple junction. Since a thermocouple junction must be covered by a metal layer or sheath at the tip, which directly contacts the surface being monitored, and a certain thermal resistance will always exist at the interface between the sheath tip and the surface being monitored, a temperature differential will normally exist between the surface being monitored and the thermocouple junction. Moreover, when a thermocouple is in contact with a surface which faces the heat source, there will be a heat flow from the source directly to the thermocouple.
As a result of all of these factors, the temperature indicated by a thermocouple can exceed the actual temperature of the wall surface being monitored by 50.degree. C. or more at the temperature levels required for the type of annealing process described above.